Transmission is the method by which computer networks send electrical signals. Data communication is concerned with the process of exchanging data electronically between two communication devices. In a local area network, all communication functions are performed through direct cables. Computers located far away are connected through a special form of data communication link. The existing telephone or telegraphic network may also be used for connecting computers located at remote locations.

Two types of signals are widely used for transmission. They are

Analog signals

Digital signals

Analog Signal Transmission

Transmission of data in analog form is done in the form of continuous waveforms. An analog signal resembles the one shown in Figure.

Information is first converted into an electrical signal known as base band electrical signal. Transmission is done by sending this base band signal directly. A special process called modulation is used in analog transmission to accommodate more than one information signal into a singlechannel. Modulation means varying anyone of the parameters of a high frequency waveformproportional to the amplitude of the base band signal. The parameters of the carrier often used forthe modulation are amplitude, frequency, and phase. A model amplitude modulated waveform isshown in Figure.

Digital Signal Transmission

Digital data transmission is the transmission of discrete data using two distinct electric states. A '1' for "on" and '0' for "off" represents these two states. Digital communication is more efficient than analog communication due to its discrete nature. Communication between computers is carried out in digital, as computers can recognize only binary data.

Digital signals also can be modulated or encoded to represent binary data. Following are some of the techniques used in current state strategies

Amplitude shift keying

Frequency shift keying

Phase shift keying

Amplitude shift keying encodes data using two signals of same frequency but different amplitudes. For example, a binary a might be represented by a 1volt signal and a binary 1might be represented by a 4 volts signal.

Frequency shift keying encodes binary data by varying the signal's frequency between two or more values. For example, the signal used for representing a binary 0 would be in one frequency and the signal for a binary 1 would be in some other frequency.

Phase shift keying encodes binary data by varying the phase of the signal to indicate a value. For example, a 180" phase-shifted signal could represent binary 1, and signal without phase shift could represents a binary 0. Figure illustrates the three keying schemes.